Nb2O5 Nanostructures as Precursors of Cycling Catalysts for Hydrogen Storage in MgH2

High operating temperatures and sluggish kinetics are major obstacles for practical applications of MgH2 as a solid hydrogen carrier. Introducing nanoscaled high-activity catalysts has been effective in improving the hydrogen cycling of MgH2. However, it remains still unclear that between nanoparticle size and morphology, which one is the decisive factor of the catalytic activity of a given catalyst. In this work, we studied this topic by taking nanostructured niobium oxide (Nb2O5) as a representative sample. Five types of Nb2O5 catalytic additives with different morphologies and nanosizes were synthesized, and their catalytic activities were compared with commercial microparticles. Our results unambiguously demonstrate that the catalytic activity of Nb2O5 is determined by the primary particle size rather than the morphology and structure because the ultrasmall Nb2O5 nanoparticles that measured ∼5 nm in size enable dehydrogenation of MgH2 starting at 165 °C after one-cycle activation. The smaller nanoparticle sizes not only enhance the reactivity of Nb2O5 but also lead to more uniform dispersion when ball-milled with MgH2, which enables in situ formation of more homogeneous and finer Nb-based active species and therefore much higher catalytic activity. This important insight will guide the design and optimization of novel high-activity catalysts for hydrogen cycling of MgH2 and other hydrogen storage materials.